Problem: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $n \neq 0$. $a = \dfrac{n + 9}{n - 6} \times \dfrac{n^2 + n - 42}{-6n - 54} $
First factor the quadratic. $a = \dfrac{n + 9}{n - 6} \times \dfrac{(n - 6)(n + 7)}{-6n - 54} $ Then factor out any other terms. $a = \dfrac{n + 9}{n - 6} \times \dfrac{(n - 6)(n + 7)}{-6(n + 9)} $ Then multiply the two numerators and multiply the two denominators. $a = \dfrac{ (n + 9) \times (n - 6)(n + 7) } { (n - 6) \times -6(n + 9) } $ $a = \dfrac{ (n + 9)(n - 6)(n + 7)}{ -6(n - 6)(n + 9)} $ Notice that $(n + 9)$ and $(n - 6)$ appear in both the numerator and denominator so we can cancel them. $a = \dfrac{ \cancel{(n + 9)}(n - 6)(n + 7)}{ -6\cancel{(n - 6)}(n + 9)} $ We are dividing by $n - 6$ , so $n - 6 \neq 0$ Therefore, $n \neq 6$ $a = \dfrac{ \cancel{(n + 9)}\cancel{(n - 6)}(n + 7)}{ -6\cancel{(n - 6)}\cancel{(n + 9)}} $ We are dividing by $n + 9$ , so $n + 9 \neq 0$ Therefore, $n \neq -9$ $a = \dfrac{n + 7}{-6} $ $a = \dfrac{-(n + 7)}{6} ; \space n \neq 6 ; \space n \neq -9 $